Many methods are known in this field for communicating between an interrogator and a transponder. An early prior art patent (U.S. Pat. No. 3,713,148) issued to Cardullo et al. describes a system in which an interrogation signal is sent at a first frequency and a transponder responds to this interrogation signal with a response signal of a second frequency. It is common in the prior art to use the interrogation signal at the first frequency as a powering signal so the transponder can operate without a power source of its own. Schuermann et al. in U.S. Pat. No. 5,053,774 describe a system and method by which a single frequency can be used for the powering of the transponder as well as bidirectional communication between the interrogator and the transponder. Schuermann et al. use a half-duplex system such that a single antenna may be used in each of the interrogator and the transponder for bidirectional communication and for powering.